Endoscopes have become widely used in the medical and industrial fields. Also, recently in the medical field, a capsule endoscope that may be swallowed by a patient has been introduced. Such capsule endoscopes are disclosed in Japanese Laid-Open Patent Application 2001-91860 and in International Patent Publication WO 01/65995 A2, and they are advantageous in that they avoid the pain associated with inserting a conventional endoscope's insertion portion.
In Japanese Laid-Open Patent Application 2001-91860, illumination means such as LED's are installed on either side of an objective lens within a transparent cover having a nearly hemispherical shape, and a portion of an object that is illuminated by the light emitting diodes is imaged by the objective lens onto an image sensor. The arrangement of components in the capsule endoscope disclosed in International Patent Publication WO 01/65995 A2 has a similar construction as that discussed above for Japanese Laid-Open Patent Application 2001-91860.
In these prior art examples, an objective lens and an illuminating means are fixed within a transparent cover having a nearly hemispherical shape and, because the radius of curvature of the transparent cover is the same at both the center of the field of view and at the periphery of the field of view, there has been an inconvenience in that the mucosa of lumen organs easily adheres to the central part of the transparent cover. Also, in the case of a transparent cover having a nearly hemispherical shape, because the radius of curvature is determined by the outer diameter of the capsule that connects to the transparent cover, there has been the inconvenience that the overall length of the capsule endoscope becomes large.
Also, no disclosure has been made concerning the performance of an objective optical system as affected by the shape of the observation target or by the focus adjustment condition of the capsule endoscope for providing an optimal observation range.